Modularized System and Method for Urea Production Using Stranded Natural Gas

ABSTRACT

A modular system and method for producing urea from stranded natural gas includes removal of foreign particulate matter to obtain a substantially homogeneous gas. The gas is processed by controlling the quality of the stranded natural gas to maintain a substantially homogenous mixture The resultant gas stream is further cleaned and compressed to a high pressure of about 3,000 psi. The resultant ammonia stream is processed in a bypass recycling loop system at 30% conversion rate at a high pressure of about 6,000 to 7,000 psi. The equipment associated with each of the process steps may be skid mounted for portability and/or contained within the footprint of a standard 48-foot flatbed trailer.

CROSS-REFERENCE TO PENDING APPLICATIONS

This application claims priority to, and the benefit of, U.S.Provisional Pat. App. No. 61/233,271, filed Aug. 12, 2009.

BACKGROUND OF THE INVENTION

The present invention relates generally to systems, methods andapparatuses for producing fertilizer and/or mixed fuels. Moreparticularly, the invention relates to systems, methods and apparatusesthat use stranded natural gas as a feedstock to produce high nitrogenfertilizers such as urea.

The market for high nitrogen fertilizers such as urea (which containsabout 46% nitrogen) continues to grow. For example, U.S. domesticconsumption of urea has experienced a 1.2% growth rate per year for thepast six years. In 2008, North American consumption exceeded 6.5 milliontons while domestic production was less than 4.5 million tons.Consequently, the balance had to be supplemented with imported product.

The growth in demand for urea stems from its versatility, portability,and capability. Urea has various uses, including use as an agriculturalfertilizer, as raw material input for production of plastics, and use bythe surfactant industry. Moreover, urea is compatible with the local andregional markets for the product. Further, urea is also beneficial dueto manufacturing cost per ton of production. Additionally, urea has anumber of advantages over other nitrogen fertilizers. For example, ureais safer to ship and handle and is less corrosive to equipment. It alsohas a higher analysis than any other dry nitrogen fertilizer.Furthermore, the high analysis means a reduced transportation andapplication cost per pound of nitrogen. It can also be applied in manydifferent ways, from sophisticated aerial application equipment tomanual hand spreading. Urea is also highly water soluble so it movesreadily into the soil. In addition, it can be used on virtually allcrops. Another benefit is that the manufacturing of urea releases fewpollutants to the environment. Urea can also be stored and distributedthrough conventional systems.

The advantages of urea relative to other fertilizers helps make urea themajor fertilizer traded in international commerce. In the very nearfuture, urea is expected to account for more than 50% of the nitrogenfertilizer in world trade. When compared to other dry fertilizers, ureahas captured more than 65% of the world fertilizer trade.

Currently, over 90% of the urea produced utilizes natural gas as thefeedstock. Over the past several years, natural gas costs have risendramatically. In some cases, a 50% increase has been realized. Duringthe winter of 2000-01, natural gas prices experienced a 400% increase.Because of natural gas prices, U.S. domestic nitrogen fertilizerproduction has dropped and imports have risen.

Urea production is natural-gas intensive. To produce one ton of nitrogenfertilizer from natural gas requires the consumption of between 20,000and 33,800 cubic feet of natural gas. Utilizing 33,800 cubic feet perton as an example, and considering each cubic foot of natural gascontains 1031 BTU's; one ton of fertilizer made from natural gascontains the equivalent of over 34.8 million total BTU's. In terms ofgasoline equivalents, this would amount to over 300 gallons of gasolineper ton of fertilizer produced. Therefore, producing urea from normalsources of natural gas (i.e., non-stranded sources) is a costlyproposition.

The use of stranded or flared natural gas sources, which areeconomically unviable for oil producers, could become a viable source offeedstock for urea production only if the quality of the incoming gasstream could be controlled and a low cost small production facilitycould be made available which does not require the high BTU content ofthe typical natural gas stream. Current global natural gas reservestotal approximately 6,100 trillion cubic feet (tcf), according to U.S.Energy Administration Information estimates. Of these, roughly half areconsidered to be “stranded,” that is, uneconomical to deliver to market.In addition, the World Bank estimates that over 150 billion cubic meters(born) of stranded natural gas are flared annually. When dealing withstranded natural gas, oil producers often find the energy, or BTUcontent, is too low; the gas is too impure to use; or, the volume is toosmall to warrant a pipeline connection to the gas infrastructure. Inaddition, the stranded gas is sometimes produced along with the oil,becoming an environmental liability. This unwanted, non-commercialby-product of oil production has become a major problem in oil fieldswhere producers have been forced to abandon well sites early, leavingvaluable reserves of domestic oil untapped.

Typically, there are three ways to deal with stranded gas: (1) ventingor flaring the gas, which contributes to air pollution without anybeneficial offsets from the gas; (2) using electrical energy tore-inject the gas, which incurs significant extra costs; and (3)shutting down oil production, which leaves valuable oil in the ground.

Another form of stranded natural gas is “associated gas,” or gas foundin association with development of large oil fields. While crude oil canbe transported to distant markets with relative ease, the practice inthe past has been to flare associated gas at the wellhead. This practicehowever is no longer acceptable due to environmental concerns and, morerecently, due to the growing economic value of these reserves in ahigh-energy price environment. Oil producers are now looking to usetechnology to capture associated gas (stranded gas) and take it toconsuming markets.

SUMMARY OF THE INVENTION

A modularized method for producing a fertilizer or a fuel from astranded natural gas feedstock includes the steps of:

-   -   i. capturing a natural gas feedstock that includes at least one        stranded natural gas feedstock from at least one stranded        natural gas source;    -   ii. removing moisture from the captured natural gas feedstock;    -   iii. removing potential disruptive inorganics and organics from        the substantially moisture-free natural gas feedstock;    -   iv. reformulating the substantially clean natural gas feedstock;    -   v. recovering a carbon dioxide (CO₂) stream from the        reformulated natural gas feedstock; and    -   vi. combining the recovered CO₂ stream with an ammonia (NH₃)        stream to form at least one of a fertilizer and a fuel.        Although the method is designed primarily for low volume        production, the equipment embodying the method may be placed in        series or in parallel with other sets of equipment embodying the        method in order to increase production volumes.

To make efficient, economical use of the stranded gas and achievefertilizer or fuel yields per volume of feedstock comparable to that ofmuch larger, conventional plants which rely upon higher quality naturalgas, quality control is especially important. For example, unlike aconventional plant that has a relatively consistent quality of gasfeedstock, the stranded or flared gas streams that are used in thismethod vary in their processing characteristics, pressures, and volumes.Rather than adjusting process parameters to accommodate the incomingfeedstock, the incoming feedstock is blended to form a substantiallyhomogenous blend (albeit a still lower quality than that of the naturalgas feedstock to a conventional plant). The homogenous blend may have,for example, a consistent BTU value or sulfur content. Similarly, themoisture removal step removes moisture to a predetermined moisturecontent, regardless of the source of the incoming feedstock. Further,processing temperatures and pressures are maintained within a desiredrange rather than changing in response to feedstock quality. A portionof the reformulating step, therefore, occurs in a temperature range ofabout 500° to 800° C. and the resultant CO₂ stream is compressed to apressure of about 3,000 psi.

Unlike the prior art systems, the method includes a bypass loop recyclesystem at 30% conversion rate run at very high pressures (about 6,000 to7,000 psi) which results in almost 100% conversion rates. The higherpressures allow for better separation of the chemical break-down duringthe creation of the ammonia. This conversion performance cannot beachieved with the same level of productivity by the high volume, lowpressure processes in common use in today's industry. These prior artprocesses run at pressures approximately one-half the pressure of theprocess described herein.

The processing of stranded and flared natural gas using this method canalso be used with modifications to generate other usable products.Unlike the prior art methods, which seek to optimize each and every stepof the process, the method according to this invention incorporates theconcept of systems “sub-optimization” developed by the American scholarand researcher Dr. W. Edwards Deming. The concept of sub-optimizationstates that a whole process (system) may result in sub-optimizedperformance by optimizing each individual sub-process (sub-system). Truesystems optimization is obtained by sub-optimizing the performance ofthe sub-system, when necessary, to achieve the optimization of the wholeor complete system. Requiring additional in-process equipment, capitalexpenditure and processing time optimizes the critical processes ofammonia/urea production, and thereby makes feasible the processing ofstranded natural gas to usable and viable products.

A urea production plant made according to this process preferablyincorporates a modular construction, another unique feature of theinvention. The plant is based on a five module configuration that isdesigned to minimize on-sight erection and start-up time/cost. The totalpackage design improves the overall reliability along with flexibility.Preferably, all of the equipment associated with each of the modules istemporarily mounted within a footprint of a standard flatbed trucktrailer. This temporary mounting may occur on the truck trailer itselfor on a concrete pad about the size of the truck trailer. Regardless ofwhether the equipment is mounted on a truck trailer (or skid-mounted)and moved, or disassembled from the pad and then moved, the plant iseasily and readily transported from one site to the next.

Modular construction minimizes the footprint of the production unitswhile maintaining ease of operation and maintenance. Modularconstruction increases the ease with which updates and modifications canbe performed as well as allowing units to be built in a central locationand shipped anywhere around the world, or manufactured in the country ofoperation using standardized plans and specifications. In addition,modularization allows for the upgrade of the production plant byreplacing specific modules when technical advances in such modules aredeveloped without affecting the other modules that comprise the wholesystem (plant). This also allows for reduced downtime for processupgrades and maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a urea production process thatconverts Stranded/Flared Natural Gas into urea. The gas is cleaned priorto being fed into a gasification unit.

FIG. 2 is a block layout of the gasification system module arranged on astandard 48-foot flatbed trailer.

FIG. 3 is a block layout of the urea conversion module arranged on astandard 48-foot flatbed trailer.

FIG. 4 is a schematic representation of the interrelationship betweenthe carbon dioxide compression component, condenser/reactor, and waterextraction and drying components of the urea conversion module and thegas stream.

FIG. 5 is a schematic representation of the carbon dioxide compressioncomponent of the urea conversion module.

FIG. 6 is a schematic representation of the pool condenser/reactorcomponent of the urea conversion module.

FIG. 7 is a schematic representation of the water extraction and dryingcomponent of the urea conversion module.

DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Theseare, of course, merely examples and are not intended to limit theinvention from that described in the claims. Well known elements arepresented without detailed description in order not to obscure thepresent invention in unnecessary detail. For the most part, detailsunnecessary to obtain a complete understanding of the present inventionhave been omitted inasmuch as such details are within the skills ofpersons of ordinary skill in the relevant art.

Current plants for the production of high nitrogen fertilizers arelarge-scale, permanent facilities that take several years to build. Tobe economically viable, these plants, and their associated processingmethods and equipment, require commercial grade natural gas atsufficient volume and pressure. Because the sources of stranded naturalgas are geographically scattered, the quality of the gas is poor, andthe volumes and pressures of the gas produced are relatively low, thegas is not a viable feedstock for these plants.

Unlike conventional plants, a production plant made according to thepresent invention can be built in about half the time. Because the costof the plant is relatively low, and because the plant makes use ofdifferent ways to treat the gas feedstock, the plant is economicallyviable to produce high nitrogen fertilizers such as urea and other mixedfuels. Further, because the plant is modularized, the plant may be sitedon a mobile pad (such as a flatbed trailer) or temporally sited on aconcrete pad and then deconstructed, moved, and reconstructed in amatter of a few months. The modularized design of the plant allows theplant to go to the sources of stranded natural gas rather than requirethose sources come to it. This makes the design ideal for use in remoterural areas that have geographically scattered or low producing wellsites, or areas that produce low quality gas or lack the infrastructurenecessary to move large quantities of gas over long distances to acentral location. Last, because of the design's modularity, the plantsare easily maintained, self-sufficient and highly automated. This lendsitself well to operating in remote well head locations.

The use of stranded natural gas for the production of urea based onutilizing approximately 30,000 cubic feet of stranded natural gas toproduce one ton of nitrogen fertilizer. Based on this relationship, thefollowing production estimates are derived:

Stranded Gas Feedstock in Yield CF/hr (000) Urea (ton/hr or TPH) 40.51.35 82.5 2.75 124.8 4.16Achieving yields using stranded gas feedstock that are comparable tothose using higher quality natural gas is a result of the unique andinventive characteristics of the method disclosed and claimed herein.Preferably, embodiments of the present invention are available in 1.35,2.75 and 4.16 TPH sizes.

Regardless of TPH size, a plant made according to this invention may beparalleled or placed in series with other like-made plants to produceelectrical power or bio-liquids (e.g., gasoline, diesel, jet fuel,fertilizers and other chemicals) in larger quantities. When compared toconventional plants, the smaller TPH size, provides many advantages,including: improved reliability; customizability; efficiency;portability; economy; compact units; environmentally friendly (meeting,for example, Environmental Protection Agency regulations and TexasCommission on Environmental Quality regulations) and operational ease.

The modular construction of the present invention also allows a user tooptimize production based on the availability of stranded natural gas ina particular field. The modular construction also allows for themovement of the plant when a field or well becomes nonproductive.

I. Urea Production

Purification Module 100

The system and process of the present invention will now be described inthe following paragraphs referring to FIG. 1.

The purification module 100 starts with filtering step 101 to reduce themoisture content of the stranded natural gas stream and obtain asubstantially water-free fuel mixture of nitrogen and hydrogen in thestoichiometric ratio of 1:3. Once the moisture content is reduced to apredetermined level, high pressure steam is introduced to heat the fuelmixture to approximately 400° C. The heated fuel mixture is passed overa catalyst to remove potential disruptive inorganics and organics fromthe mixture. The catalyst converts nonreactive organic sulfur compoundsto hydrogen sulfide. Hydrogen sulfide is removed by passing the mixtureover a bed of zinc oxide particles in the desulferizing step 102. Thezinc oxide particles absorb the hydrogen sulfide. The purified gasstream is then ready for the reforming module 200.

Filtering step 101 or desulferizing step 102 may be proceeded by ablending step (not shown) in which two of more different strandednatural gas streams are blended together to form a single substantiallyhomogeneous stream. The importance of creating a homogenous feedstockwhen using biomass to produce urea is discussed in our earlierinternational application PCT/US2009.053537, titled “Modularized Systemand Method for Urea Production Using a Biomass Feedstock,” published asWO/2010/019662 on Feb. 18, 2010, the content of which is herebyincorporated by reference.

Blending the streams to produce a single stream is important when usingstranded natural gas because the gas produced by different well sitesmay have different processing characteristics, such as the amount ofmoisture, sulfur or BTU content. Failing to provide downstream moduleswith a consistent quality of gas (regardless of whether that quality isrelatively high or low) makes it difficult to control the processesassociated with those downstream modules and produce an end producthaving consistent quality. Unlike prior art processes, which require acertain quality of natural gas, the process described herein makes useof whatever quality of gas is available. For this reason, strandednatural gas is acceptable as a feedstock and could, if desired, beblended together with a higher quality, commercial-grade natural gasstream and processed.

Reforming Module 200

The reforming module 200 starts with a primary reforming step 201 inwhich the purified gas stream from Module 100 flows into indirectlyheated tubes filled with nickel containing a reforming catalyst. Theindirectly heated tubes raise the temperature of the gas stream to about500 to 800° C. In primary reforming step 201 the reaction is controlledto achieve only a partial conversion of approximately 65% based on themethane feed from module 100. In a subsequent secondary reforming step202 the partially converted gas stream is passed through a refractorylined reaction vessel with nickel catalyst and mixed with a controlledamount of combustion air. The combustion of the partially converted gasstream further raises the temperature to approximately 1,200° C. Thecombusted gas stream then flows through another catalyst layer where theoutlet temperature is lowered to approximately 1,000° C. and theresidual methane is less than 0.5%. The outgoing reformed gas stream,which is compressed to at least 206 bar (about 3,000 psi), is then readyfor shift conversion.

Shift Conversion Module 300

Shift conversion module 300 uses a water-gas shift reaction. The carbonmonoxide (CO) serves as a reducing agent for water to yield hydrogen (H)and carbon dioxide (CO₂). Module 300 not only produces more H forammonia module 400 but also converts the CO to CO₂ which will be used asa chemical component in the urea production module 500.

Shift conversion module 300 begins with step 301, high temperature shiftconversion, which utilizes an iron-based catalyst with an additional 5to 10% chromic oxide. Steam is introduced to the incoming reformed gasstream and the temperature of the reaction is held to a range of about300 to 500° C. This is a controlled process and is dependent on theratio of CO/CO₂.

Low temperature shift conversion step 302 utilizes an iron-chromium andcopper-zinc catalyst that is active at a temperature range of about 320to 360° C. Step 302 furthers the reaction and also works to absorbresidual sulfur (<0.1 ppm) to prevent poisoning of the catalyst. CO₂ isstripped 303, 303 a, compressed at approximately 206 bar (about 3,000psi) and flowed to the urea conversion module 500.

Ammonia Module 400

Ammonia module 400 involves a purification process using a simplereversal of the primary reforming step 201 to reduce carbon oxides toless than 10 ppm. A nickel catalyst, at a pressure of about 25 to 35 bar(about 360 to 510 psi), controlled at temperature between about 250 to350° C. is utilized in the methanation process step 401. The processedgas exiting step 401 is then compressed in syngas compression step 402at approximately 150 to 175 bar (about 2,175 to 2,550 psi) and flowed tothe ammonia convertor loop 403. The ammonia convertor loop 403 is usedto continuously recycle the gas over an iron catalyst using a H₂recovery feed 404, 404 a. A refrigeration loop 405 is utilized to coolthe gas after passing over the catalyst which allows for the pureammonia (NH₃) to condense out. Ammonia converter loop 403 is a bypassrecycling loop at a high pressure range of between about 410 to 485 bar(about 6,000 to 7,000 psi) and results in about a 30% conversion rate.

Urea Conversion Module 500

Urea production module 500 is described in our previously mentionedinternational application. Urea conversion module 500 receives thecompressed CO₂ from step 303 a and the NH₃ from step 403 and flows thecompressed CO₂ and NH₃ to a pool condenser step 501 (see FIGS. 4, 5 &6). NH₃ and CO₂ are introduced into the pool condenser 501 a by ahigh-pressure ammonia pump and a carbon dioxide compressor (see FIG. 6).The CO₂ and NH₃ gas streams are flowed counter-current to one another inorder to improve the overall reaction within the pool condenser 501 a.About two-thirds of the urea conversion takes place in the poolcondenser 501 a. After the pool condenser 501 a the remaining gases andurea-carbamate liquid enter the vertical pool reactor 501 b in which thefinal urea formation takes place. Any un-reacted carbamate may be routedto a scrubber/recycler 501 c for reintroduction to vertical pool reactor501 b.

The resulting urea slurry or solution is sent to a drying step 502 wherewater is removed (see FIG. 7). Water extraction occurs by way of vacuumextraction 502 a. The remaining urea melt is then sent to a drying unitor film dryer (granulation) 502 b where it is further dried using a filmdrying process to result in a final product to be stored. The waterremoved from vacuum extraction 502 a, and from film dryer 502 b, ispreferably recycled through the system but may be treated anddischarged.

II. Modular Arrangement

Referring now to FIGS. 2 and 3 (and referring back to FIG. 1), the ureaproduction process may be a modularized process, with process steps 101to 102 comprising purification module 100, steps 201 to 202 comprisingreforming module 200, steps 301 to 303 comprising shift conversionmodule 300, steps 401 to 405 comprising the ammonia module 400, andsteps 501 to 502 comprising a urea conversion module 500. In FIGS. 2 and3, the various pieces of process equipment associated with each modulehave been mapped to the corresponding process steps of FIG. 1.

The purification module 100, reforming module 200, shift conversionmodule 300, ammonia module 400, and urea conversion module 500 may bearranged for turn-key operation preferably on a concrete pad (if asemi-permanent installation is required) or on standard 48-foot flatbedtrailers T, respectively. If a smaller size flatbed trailer is used, itmay be necessary to divide the individual component parts of the module100, 200, 300, 400, or 500 into two or more flatbed trailers withappropriate connections being provided.

Each module 100, 200, 300, 400, and 500 is preferably skid-mounted forease of offloading to a remote site. A portable power plant P may beprovided to power one or more of the modules 100, 200, 300, 400, 500.Although the process flow and interconnections between variouscomponents are not shown in FIGS. 2 and 3 (as well as in FIGS. 4 to 7),a person of ordinary skill in the art would recognize the flow patternand the types of connections required for various process components.

Referring now to FIGS. 6 to 7, an alternate preferred embodiment of ureaconversion module 500 is shown which may be arranged so as to fit withinthe footprint of a standard 48-foot flatbed trailer T (or concrete pad).Similar to FIGS. 2 to 5 above, the various pieces of equipmentassociated with the urea conversion module 500 have been mapped to thecorresponding process steps of FIG. 1.

While a modular system and method for urea production has been describedwith a certain degree of particularity, many changes may be made in thedetails of construction and the arrangement of components and stepswithout departing from the spirit and scope of this disclosure. A systemand method according to this disclosure, therefore, is limited only bythe scope of the attached claims, including the full range ofequivalency to which each element thereof is entitled.

What is claimed is:
 1. A method for producing a fertilizer or a fuelfrom a stranded natural gas feedstock, the method comprising the stepsof: i. capturing a natural gas feedstock that includes at least onestranded natural gas feedstock from at least one stranded natural gassource; ii. removing moisture from the captured natural gas feedstock;iii. removing potential disruptive inorganics and organics from thesubstantially moisture-free natural gas feedstock; iv. reformulating thesubstantially clean natural gas feedstock; v. recovering a CO₂ streamfrom the reformulated natural gas feedstock; and vi. combining therecovered CO₂ stream with a NH₃ stream to form at least one of afertilizer and a fuel.
 2. A method according to claim 1 furthercomprising the step of blending the captured natural gas feedstock priorto the reformulating step to form a substantially homogenous blend.
 3. Amethod according to claim 2 wherein the captured natural gas feedstockincludes at least two stranded natural gas feedstocks each having atleast one different processing characteristic.
 4. A method according toclaim 2 further comprising the blending step resulting in a consistentBTU value for the homogeneous blend.
 5. A method according to claim 2further comprising the blending step resulting in a consistent sulfurcontent for the homogeneous blend
 6. A method according to claim 1wherein the moisture removal step removes moisture to a predeterminedmoisture content.
 7. A method according to claim 1 further comprising aportion of the reformulating step occurring in a temperature range ofabout 500° to 800° C.
 8. A method according to claim 1 furthercomprising the reformulating step including the sub-step of compressingthe resultant CO₂ stream to a pressure of at least about 3,000 psi.
 9. Amethod according to claim 1 further comprising the step of processingthe NH₃ stream in a bypass recycling loop.
 10. A method according toclaim 9 wherein the bypass recycling loop operates at a high pressurerange of between about 6,000 to 7,000 psi and results in about a 30%conversion rate.
 11. A method according to claim 1 wherein equipmentembodying the method is placed in series with at least one other set ofequipment embodying the method.
 12. A method according to claim 1wherein equipment embodying the method is placed in parallel with atleast one other set of equipment embodying the method.
 13. A methodaccording to claim 1 wherein all equipment associated with at least oneof the steps (i) to (vi) is substantially immediately portable between afirst and second stranded natural gas source site.
 14. A methodaccording to claim 13 wherein all equipment associated with at least oneof steps (i) to (vi) is temporarily mounted within a footprint of astandard flatbed truck trailer.
 15. A method according to claim 13wherein all equipment associated with at least one of the steps (i) to(vi) is skid mounted.
 16. A method according to claim 1 wherein allequipment associated with at least one of the steps (i) to (vi) istemporarily positioned at a location in fluid communication with the atleast one stranded natural gas source.
 17. For producing a fertilizer ora fuel, a system comprising: a purification module; a reformulationmodule; a shift conversion module; an ammonia module; and a ureaconversion; at least one of the modules being substantially immediatelyportable between a first and second stranded natural gas source site; anatural gas feedstock including at least one stranded natural gasfeedstock being inputted to the purification module.
 18. A systemaccording to claim 17 wherein the purification module provides asubstantially homogenous blend of two or more stranded natural gasfeedstocks to the reformulation module.
 19. A system according to claim17 wherein a resultant gas stream from the reformulation module iscompressed to a pressure of at least about 3,000 psi.
 20. A systemaccording to claim 17 wherein the ammonia module further comprises abypass recycling loop operating in a high pressure range of betweenabout 6,000 to 7,000 psi and resulting in about a 30% conversion rate.